Within the field of watch-making, a conventional architecture is used to make movements, which may also include striking mechanisms. This striking mechanism can be actuated at well defined times to indicate a programmed alarm or minute repeaters. In a conventional embodiment of a striking watch, the striking mechanism may include a gong. The gong is generally a metal wire of circular shape, which surrounds part of the watch movement inside a watch frame. The gong is fixed to a gong-carrier, which is integral with a bottom plate of the watch. The gong vibration is generated by the impact of at least one hammer against the gong at determined times.
In the case of a striking watch, such as a musical watch, the sound is produced by the vibrating tongues of a pin-barrel. The pin-barrel tongues are made together with a heel of the pin-barrel, which is mounted on the watch plate. For producing music, for example at programmed time periods, the pin-barrel tongues are raised and then released by pins secured to a rotating disc or cylinder. Each tongue can therefore bend via the action of a corresponding pin of the cylinder or disc, and as soon as said tongue is released, it oscillates mainly at its first natural frequency. The musical pin-barrel is enclosed in the watch case. Consequently, the vibrations generated by the actuated tongues are transmitted to the external parts of the watch.
The external parts of the watch are, for example, the middle part, the bezel, the crystal or the back cover of the case. When a sound is produced either by a gong struck by a hammer, or by one or more vibrating pin-barrel tongues, these external parts are capable of radiating the produced sound into the air. In a conventional striking watch, the acoustic efficiency, based on the complex vibro-acoustic transduction of the external parts, is low. In order to improve and increase the acoustic level perceived by the user of the striking watch, the material, geometry and boundary conditions of said external parts must be taken into account. The configurations of these external parts are also dependent upon the aesthetic appearance of the watch and operating stresses, which may limit adaptation possibilities.
It is known in watchmaking technology to use an acoustic type membrane, which is dedicated to vibro-acoustic transduction, in a watch and particularly an electronic watch. To activate this type of membrane in an electronic watch, a piezoelectric element is, for example, placed on the membrane to cause it to vibrate. To prevent the acoustic radiation from the membrane from being lost in the watch, which must be sealed, a double back cover can be provided for the watch case, which must be open towards the exterior. In such case, the back cover of the watch case has one or more apertures for the transmission of sound from the vibrating membrane.
With this type of design for an electronic watch with an acoustic membrane, problems often arise relating to the sealing and corrosion of said membrane. The first natural vibration frequency of this membrane, which is the efficient radiation mode, is within the useful acoustic frequency band, whereas its second natural frequency, which is an inefficient mode, must if possible be outside this audible band. Since the membrane is usually made of steel, the first and second natural vibration frequencies do not fulfil the aforementioned conditions in an optimum manner. Moreover, rapid damping is observed, which is a drawback.
In a standard striking watch, which is, for example, fitted with an acoustic membrane, the membrane is sandwiched between part of the middle case and the back cover of the watch. In the case of a luxury watch, the back cover may be made of a precious material, such as gold. A difference in electrochemical potential may occur on contact between the membrane, which is generally made of steel, and the gold back cover, especially in a humid environment. This is liable to contribute to corrosion of said membrane where it is in contact with the gold back cover, which is another drawback.